This is a followup to r194536, which changed the pair copy constructor to be
trivial in C++03, thus making it trivial in both C++03 and C++11.

This patch allows one to opt-in/out of this decision with a macro.  You can
choose to have the pair copy constructor always be trivial, or always be
non-trivial.  The flag controlling this is now _LIBCPP_TRIVIAL_PAIR_COPY_CTOR.

The client can define this flag to 1, and the pair copy constructor will be
trivial (when possible of course), or to 0, and the pair copy constructor will
be nontrivial.

Default settings for this flag are set in <__config> (as usual).  With this
commit the default is _LIBCPP_TRIVIAL_PAIR_COPY_CTOR=1 for all platforms
except __APPLE__, which defaults to _LIBCPP_TRIVIAL_PAIR_COPY_CTOR=0.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@194742 91177308-0d34-0410-b5e6-96231b3b80d8